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Some ten years ago E. L. Anderson, author of Modern Horsemanship, wrote me, complaining that the Spanish walk and trot disturbs the fineness of a horse's mouth, so necessary for the piaffer and the passage. I replied that this is certainly the fact. In the passage and the piaffer, the exertion being less than in the Spanish walk and trot, the rhomboideus acts more strongly than the mastoido-humeralis. In the Spanish walk and trot, which involve greater exertion, the conditions are reversed, and the mastoido-humeralis acts the more strongly. But it is the action of the first of these muscles, the rhomboideus, that gives the more sensitive contact against the hand.
